Fault slip beyond a barrier on a transform plate boundary

A strike‐slip plate boundary is considered with non‐uniform strength. Depth‐dependent slip on the fault surface is assumed to occur aseismically in the lower fault section, in response to a basal shear stress, and seismically in the brittle upper section. The upper fault section acts as a barrier, preventing aseismic slip from reaching shallower depths. If, however, friction decreases in the proximity of the Earth's surface, brittle fracture may occur beyond the barrier. It is found that, when friction in the uppermost fault section exceeds the applied shear stress, fault slip occurs there with a smaller amplitude than at larger depth. This may explain the discrepancies, which have been sometimes reported, between fault offsets observed at the Earth's surface and fault offsets inferred from seismograms by using simpler dislocation models.